1. Field of the Invention
This invention relates to laser scanners and especially to a laser scanner using short wavelength laser radiation.
2. Description of the Prior Art
Photolithography is commonly employed to produce repeatable patterns on devices such as integrated circuits, integrated circuit masks, flat panel displays, and printed circuit boards. A conventional photolithography process coats a workpiece with a layer of photoresist and illuminates selected regions of the photoresist with light that changes the properties of the illuminated regions. The photoresist layer is then developed and either the illuminated regions or not-illuminated regions (depending on the type of photoresist) are removed to leave a patterned layer covering portions of the workpiece. The workpiece is then subjected to a process such as etching where the covered portions of the workpiece are protected from the process.
A laser scanner is a photolithographic device which scans one or more focused and spatially modulated laser beams in a series of scan lines covering a layer being patterned. Whether a laser scanner illuminates a region depends on the laser beam's intensity as the beam scans passes the region. The precision of a laser scanner in selecting regions illuminated depends on the accuracy of modulation of the laser beam, the sharpness of the focus of the laser beam, the precision with which the laser beam moves across the layer being patterned, and synchronization between modulation and movement of the laser beam.
A typical scanner includes a laser, a modulator, scan optics, and a precision stage. The laser generates a collimated light beam which in a multi-beam system is split into an array of separate collimated sub-beams. Typically, the laser beam is ultraviolet light of wavelength e.g. 363.8 nm (nanometers), or 325 nm. Modulation of the array of beam changes the intensities of the individual sub-beams independently, typically turning sub-beams on and off; grayscale (intensity) control can also be employed.
Scan optics, including a rotating reflective polygon or other scanning device and a scan lens, forms an image of the beam or array and sweeps that image across a scan line in an image plane of the scan optics onto the surface of the workpiece to be exposed, which is held on an X-Y stage. The stage precisely moves the workpiece approximately perpendicular to the scan line direction. Movement of the workpiece can be continuous during scanning or may only occur during the dead time between scan lines. As the image sweeps across the scan line, sub-beams in the beam are turned on and off to control which regions within the scan line at the surface of the device are illuminated.
For examples of laser scanners see U.S. Pat. No. 5,255,051, issued Oct. 19, 1993, to Paul C. Allen, U.S. Pat. No. 5,327,338, issued Jul. 5, 1994, to Paul Allen, et al., and U.S. Pat. No. 5,386,221, issued Jan. 31, 1995, to Paul C. Allen, et al., all incorporated herein by reference.
A typical application of such laser scanners is, as described above, for photolithography. Certain semiconductor photolithography applications require formation of very small size features. A demand for higher mask patterning resolution, i.e. equipment for forming the masks typically used to fabricate integrated circuits, requires either higher numerical apertures or shorter wavelengths. Commercially available photolithography equipment has a numerical aperture of the lens system of less than 0.80, approximately the limit of what is practical with light (ultraviolet) wavelengths. Remaining avenues for resolution improvement require use of shorter wavelengths. However, as described above, typical wavelengths used in the prior art are in the 350 nm region. It is not possible simply to obtain a commercially available compact continuous wave laser capable of efficiently operating at shorter wavelengths with powers above approximately 1 Watt. Pulsed solid-state lasers have been demonstrated with powers greater than 1 Watt. Hence, there is a need for photolithography equipment capable of operating with a pulsed short wavelength source for forming very small size features, i.e. features of 300 nm or less in size.